This invention relates to olefin polymerization catalysts and more particularly relates to use of supported propylene polymerization catalysts in gas-phase polymerization in combination with a chloride-free co-catalyst system.
Gas-phase polymerization of olefins, especially propylene, has become an attractive commercial process. In gas-phase polymerization, gaseous olefin monomer is contacted with a catalyst system to form solid polymer without solvent or diluent. Catalyst systems useful in gas-phase polymerization must be very active. Because the forming polymer is not in contact with a solvent or diluent, such as paraffinic hydrocarbon, there is no inherent removal of hydrocarbon-soluble polymer products such as amorphous or atactic material. Therefore, catalyst systems used in gas-phase polymerization also must produce relatively low amounts of such amorphous or atactic material.
Typically, polymer produced from a gas-phase reactor is in the form of a dry powder. Sometimes it is advisable to treat such powder with a flowing gas-stream such as nitrogen containing a catalyst deactivation agent such as water, air, oxygen, alcohol, ammonium compounds, carbon oxides and the like to deactivate catalyst residues remaining in a polymer powder. If substantial amounts of corrosive catalyst residues remain in the powder, expensive stabilization agents must be incorporated within the polymer to avoid corrosion of fabrication equipment. Chlorine compounds in polymers such as polypropylene are noted as creating corrosivity problems. Although all chlorine compounds do not exhibit the same corrosivity problems, a test for "active chloride" shows levels of chlorine-containing compounds which are associated with corrosivity. A common component in olefin polymerization catalyst systems are alkyl aluminum chlorides such as diethylaluminum chloride. A method to avoid active chloride is to minimize, or preferably eliminate, incorporation as such alkylaluminum chlorides in catalyst systems.
As noted above, gas-phase polymerization systems show considerable commercial advantage. However, it has been found that catalyst systems generally useful for slurry or bulk polymerization may not be used successfully in gas-phase polymerizations. The present invention demonstrates this fact. Olefin polymerization using a titanium-containing compound in combination with alkylaluminum compounds are well known as Ziegler-Natta catalysts. There are many variations of this concept in the art with a variety of forms of titanium compounds, alkylaluminum compounds and many varieties of modifiers and promoter packages. Various titanium compounds supported on magnesium-containing compounds have been reported including U.S. Pat. Nos. 3,901,863 and 4,227,370, all incorporated by reference herein.
Co-catalysts incorporating secondary amines have been disclosed in certain contexts. For example, U.S. Pat. Nos. 4,094,818, 4,224,181 and 4,224,182 describe the use of hindered amine compounds such as 2,2,6,6-tetramethylpiperidine as one component in a cocatalyst system. U.S. Pat. Nos. 4,431,570, 4,431,571 and 4,431,572, all incorporated by reference herein, describe catalyst systems in which comminuted supported titanium-containing catalyst component is retreated with a mixture containing a titanium halide, at least one organic acid ester and, optionally, a chlorocarbon or a haloalkylchlorosilane.
Olefin polymerization catalysts, especially propylene polymerization catalysts, supported on a magnesium-containing compound and containing an electron donor compound such as an alkyl aromatic acid ester have been described widely.